This paper presents a case study that describes the selection, development, engineering and construction of a process plant for the biological oxidative pre-treatment of a refractory pyrite-arsenopyrite gold ore flotation concentrate. Prefeasibility and feasibility study resource modeling, metallurgical testwork, and environmental and engineering studies are outlined. Options considered for oxidation of the ore, which examine cost, risk, operability, recovery, engineering and environmental aspects are compared, with particular reference to pressure and biological oxidation. The BIOX biological oxidation process was selected as the preferred oxidation process and the flow sheet for this process is presented. Selection of the BOIX process resulted in special requirements for the feed preparation circuit, residue treatment and disposal, materials of construction and engineering. Use of a biological process in ore treatment also introduces particular process and engineering risks the management of which are discussed.